Strain gage-based platform scales may employ a single sensor in conjunction with force-redirecting structure to isolate the sensor from extraneous loads and compensate for loads which are applied off-center, or they may employ multiple sensing elements. One type of low profile multiple sensing beam platform scale is disclosed in U.S. Pat. No. 4,261,429.
The use of multiple sensors permits the system to be designed for minimum height, which is advantageous in weighing applications on existing counter tops which are designed at a height for maximum convenience. One such application is in the food service industry, where individual ingredient portions must be frequently and accurately weighed. Such application also requires that the unit be splash proof and washable, to maintain sanitation and scale accuracy. The weighing of products which may be substantially above or below room temperature introduces the possibility of thermal distortions of the platform, which must be minimized to maintain scale accuracy.
Furthermore, in applications such as the food service industry, where the capacity of the scale is relatively low but the required degree of accuracy is relatively high, it is important to protect the sensing mechanism from extraneously directed loads or excessive loads. These extraneous loads include those applied upwardly to the platform, such as when the scale is lifted.
Finally, it is desirable that the weighing system be designed so that the capacity or sensitivity of the system can be changed or damage repaired while retaining a maximum number of common components.
These objects have been achieved by the present invention. Protection of the sensing beams from lateral or upward forces is achieved by the use of a readily compressible resilient grommet or pad through which all downward and lateral loads are applied. One of such grommets is interposed between each of the sensing beams and the platform. The platform is maintained in proper lateral registry will the remaining structure by means of locating pins fixed to the underside of the platform and normally received in a central bore in each of the grommets. Thus, extraneous lateral forces are absorbed by the compressible grommet. Similarly, upward movement of the platform merely causes the locating pins to slide upwardly out of the grommet, there being no connection between the platform and the sensing beam under such loading conditions.
The full enclosure of the system is achieved by an assembly of the platform and a floating metallic lower enclosure plate secured to the platform. The lower enclosure plate has four openings through which each of the scale-supporting feet extend, but such openings are sealed by thin rubber diaphragms secured to the edges of each opening and to the feet. Such diaphragms are incapable of absorbing or transmitting any normally-encountered loads from the platform to the remaining structure, thereby assuring that all forces to be measured are transmitted from the platform through the resilient grommets to the sensing beams. One of the feet may be made hollow so that the electrical connections for the strain gages can enter the enclosed scale from below, without significantly compromising the washability or splash-proof features of the scale.
The thin sheet metal platform readily dissipates heat, and the lateral compressibility of the grommets aid in absorbing thermally-induced expansion or contraction of the platform.
Adjustably positioned overload stops are provided in the form of set screws in the frame which engage the underside of a sensing beam extension plate after the extension plate and sensing beam have been downwardly deflected by an applied load which exceeds a predetermined value. In this way, any further loads are safely by-passed around the sensing beam.